1. Field of the Invention
The invention is in the field of organic chemistry. The invention relates to an efficient process for transforming commercially available (3R,3′R,6′R)-lutein containing 5–7% (3R,3′R)-zeaxanthin to anhydroluteins (dehydration products of lutein) in an alcohol at elevated temperatures and subsequent conversion of the latter to (3R)-β-cryptoxanthin as the major product and (3R,6′R)-α-cryptoxanthin as the minor product.
2. Related Art
A process for converting commercially available (3R,3′R,6′R)-lutein to a mixture of (3R,6′R)-α-cryptoxanthin, (3R)-β-cryptoxanthin, and (3R,6′R)-anhydrolutein I ((3R,6′R)-3′,4′-didehydro-β,γ-caroten-3-ol), (3R,6′R)-2′,3-anhydrolutein II ((3R,6′R)-2′,3′-didehydro-β,ε-caroten-3-ol), and (3R)-3′,4′-anhydrolutein III ((3R)-3′,4′-didehydro-β,β-caroten-3-ol) in one synthetic step by allylic deoxygenation with a strong acid and a hydride ion donor was described by Khachik in U.S. patent application No. 60/220,995. The chemical structures of these carotenoids are shown in Scheme 1.
Khachik also described a two-step alternative process. The first step converted (3R,3′R,6′R)-lutein to a mixture of anhydroluteins I, II, III at room temperature with an acid. In the second step, the isolated anhydroluteins were converted to (3R,6′R)-α-cryptoxanthin and (3R)-β-cryptoxanthin with a strong acid and a hydride ion donor.
As described by Khachik, the acid-catalyzed dehydration of (3R,3′R,6′R)-lutein in a homogenous phase in a variety of solvents such as ethers (tetrahydrofuiran, tert-butyl methyl ether), chlorinated solvents (dichloromethane, chloroform, 1,2-dichloroethane), acetone, and toluene at ambient temperature leads to the formation of considerable amount of Z(cis)-isomers of anhydroluteins. In addition, under the conditions disclosed in '995 application, anhydrolutein I is the major product and anhydroluteins II and III are the minor products. Because anhydrolutein III is the precursor to (3R)-β-cryptoxanthin in the ionic hydrogenation step, a higher concentration of this carotenoid relative to anhydroluteins I and II is preferred. Therefore a modified procedure for the dehydration of (3R,3′R,6′R)-lutein is needed that can produce anhydrolutein III as the major product and at the same time significantly reduce E/Z(trans/cis)-isomerization of anhydroluteins. Such a procedure must also demonstrate that a mixture of anhydroluteins with high concentration of anhydrolutein III can be transformed into a mixture of all-E-cryptoxanthins with a high concentration of all-E-(3R)-β-cryptoxanthin. Since (3R)-β-cryptoxanthin is a precursor of vitamin A, a higher concentration of this carotenoid in the final product is desirable.
